nan·o·brew·e·ry (noun): A scaled-down microbrewery, often run by a solo entrepreneur, that produces beer in small batches.

in a recent article, the blog Top Fermented highlighted the trend towards nanobreweries, offering this definition: "There’s no good definition, but what seems to be settled upon is
that you’re making an amount of beer, per batch, that is considerably
less than one would expect from a production brewery: half-barrel or
one-barrel systems are common and sometimes even less."

Hess Brewing Company, which calls itself "San Diego's first nanobrewery," puts the total output of nanobreweries at somewhere between 10 and 75 gallons.

A 2009 article on msn.com suggested that nanobrewing has emerged as part of the larger trend in food towards locavorism:

For years, microbrewers — independent beer-brewing operations whose
threshold of production is less than 15,000 barrels a year — defined
the small-batch experience. More recently, though, the drive for local
identities and products, and a reawakening of Americans’ sense of the
neighborhood as resource, have led to the rise of nanobreweries, run by
a handful of people operating out of very limited neighborhood spaces
on a very limited budget.

Unlike microbreweries, which are the thriving results of business
models, nanobreweries are more entrepreneurial, with their creators
doing it either as a high-profile hobby or as a stepping-stone to
creating a bigger company. Either way, nanobrewing reflects the impact
of neighborhoods, as these mavericks create a brew — and a following —
one trench drain, one fermentation tank at a time.

The U.S. Department of the Treasury offers its own rough definition, while reminding budding nanobrewers that no matter how small their beer business, they are subject to federal taxation:

Nano-breweries, which we define as very small brewery operations, are
springing up across the country. Nano brewing is a result of the steady
appeal for craft-brewed beers and the beneficiary of the growing home
brewing movement. We issue this advisory as a reminder that any beer
produced for sale by home brewers is not exempt from Federal excise tax
payment.